1. Field
The subject invention relates to processing of substrates and, more specifically, for systems for forming thin films over substrates to produce devices, such as solar cells.
2. Related Art
Vacuum processing systems are used to fabricate hard-drive disks, semiconductor computer chips, solar panels, and the like, from substrates made of materials such as semiconductor wafers, glass, stainless steel, etc. Typically, the vacuum processing systems include several substrate chambers that perform various processes that modify the substrate by performing deposition, cleaning, etching, heating/cooling, etc., on the substrate. Deposition of films is generally accomplished using, e.g., physical vapor deposition (PVD) or chemical vapor deposition (CVD). PVD can be performed using, e.g., sputtering or evaporation systems. Sputtering process can be controlled relatively well and thin films formed using sputtering sources can be of high quality and uniformity. However, sputtering sources are relatively expensive and target utilization is relatively low. On the other hand, evaporation systems are relatively of low cost and high utilization, albeit using current technology they are more difficult to control to form films of precise thickness and uniformity.
Fabrication of solar cells is a recent emerging field which utilizes thin film technologies. There are several basic forms of solar cells, including c-Si, a-Si:H, n-Si:H, CIS/CIGS/CIGS-S, CdTe, GaAs and Organic or Dye Sensitized devices. There are many layer combinations that comprise modern cells, many of which may be fabricated using thin film fabrication techniques. For example, absorber layers, low resistivity rear electrodes, high resistivity intermediate or buffer layers and high optical transmission moderate resistivity window layers are essential components in the fabrication of solar cells. In order to tailor such layers to achieve requisite results on specified figures of merit, such as Voc, Isc, Fill Factor, conversion efficiency and numerous other parameters, precise atomic concentrations of materials must be deposited.
FIG. 1 illustrates schematically an evaporation system of the prior art. A feed roll provides a sheet of flexible substrate, which is collected by the end roll. As the substrate rolls, it passes over a series of evaporators. The evaporators inject the material to be deposited onto the substrate, in the proper sequence for generating the proper sequence of layers.
One problem with such systems is that it is impossible to stop deposition of a material part of the way through depositing a layer. Also, it is difficult to control the temperature uniformity, film uniformity, and reaction uniformity on such large webs. The use of webs causes the systems to be very large, such that it impacts the scale up, which is limited to large step function changes. Furthermore, when any part of the system is down, the entire system must be stopped, causing a large drop in output. Furthermore, the large substrates inherently desorbs significant amount of water, which effects the cell efficiency. Degas is also very difficult with web processing. Also, it is difficult to co-deposit material from different conventional large evaporators.
In addition to controlling the formation of the various layers, in various solar cell structures, such as in CIGS, the material concentration in each layer may have a controlled gradient. Precise control of each such gradient can help in achieving a higher conversion efficiency of the fabricated cell. However, using roll to roll technology it is impossible to generate gradients in the various layers. The National Renewable Energy Laboratory (NREL) has published on its website its equipment used for CIGS research. The diagram published on the NREL website is shown in FIG. 1A and is described on NREL's website as: “Copper Indium Gallium Diselenide cluster tool indicating the functions of its eight ports.”